It is known that internal resistance of a fuel cell is affected by the degree of dryness of an electrolyte membrane of the fuel cell. More specifically, if the water content in the fuel cell becomes insufficient and the electrolyte membrane dries (which is a state known as “dry-up”), the fuel cell internal resistance increases and the output voltage of the fuel cell decreases. In order to operate the fuel cell efficiently, the water content in the fuel cell should be controlled in an optimum state. The water content in the fuel cell has a correlation with an impedance of the fuel cell. Thus, the water content in the fuel cell can be found indirectly, for example, by measuring an impedance of the fuel cell through an AC impedance method. Patent Document 1 indicated below discloses a technique in which an impedance of a fuel cell is measured by superimposing a high frequency signal and a low frequency signal onto an output signal of the fuel cell, and after that, whether dry-up occurs or not is determined using an impedance in a high frequency range and an impedance in a low frequency range, which impedances are extracted from the above-measured impedance.